feat: add theme system, fonts, graph styling, rename Overview→Tasks

- Terracotta (light) and Carbon (dark) themes with toggle
- Inknut Antiqua headings, DM Sans body
- Dot grid background on EntityGraph and GraphBackground
- Theme-adaptive graph colors on EntityGraph
- Art Nouveau chat styling (borders, underlines, blockquote quotes)
- Bullet point styles in chat prose
- Task goal in header, rename Overview→Tasks, New Task labels
- Logo uses var(--primary) for theme awareness

web/src/lib/components/ConfigBackground.svelte

@@ -1,6 +1,6 @@
 <script lang="ts">
   import { onMount } from 'svelte'
-  import { mode } from 'mode-watcher'
+  import { getTheme } from '$lib/stores/theme.svelte'
 
   interface Particle {
     x: number
@@ -65,7 +65,7 @@
     if (!ctx) return
 
     const t = ts / 1000
-    const dark = mode.current !== 'light'
+    const dark = getTheme() !== 'light'
 
     ctx.setTransform(dpr, 0, 0, dpr, 0, 0)
     ctx.clearRect(0, 0, w, h)